gcli.css uses box-sizing in addition to using -moz-box-sizing. Unprefixed box-sizing isn't supported by Firefox yet and causes console whines. These console whines interfere with browser_webconsole_bug_603750_websocket.js when I run the test locally.
Please don't use unsupported CSS properties in CSS files that are part of the browser itself. Please rely on whoever unprefixes the property in the style system to unprefix instances of use throughout the product when the time comes.